7. PROJECT SUMMARY / ABSTRACT Oropharyngeal candidiasis can evolve into serious systemic infections with high morbidity and mortality rates in post-surgical and immunocompromised patients. A thorough understanding of its immunopathogenesis is critical to improve therapies that mitigate or prevents this disease. It is known that IL-17A provided by innate and CD4-dependent adaptive immune responses is key to controlling Candida albicans (CA) invasion and spread. What remains unknown is the role played by mucosal antigen presenting cells, in particular Langerhans cells (LCs), on the development of compensatory IL-17A-producing CD8+ T cell populations (Tc17) that are potentially protective against oral candidiasis. At homeostasis, lack of LCs or MHC-II presentation on LC, expands the Tc17 population. Our long-range goal is to identify and characterize the response to opportunistic microorganisms by components of the local immune that ultimately protect individuals from diseases at oral mucosal surfaces. We have preliminary data that suggests that absence of LCs and MHC-II presentation on LCs results in the secondary clonal expansion of Tc17, Tc1 and Treg cells at homeostasis. Our current objectives are to determine how LCs control the numbers of mucosal Tc17 cells in an MHC-II dependent manner and the extent of protection against CA when Tc17 are present. We hypothesize that intraepithelial LCs regulate the numbers of Tc17 as a compensatory source of IL-17A and a reservoir of IFN-?-producing Tc1 contributing to an efficient barrier against chronic CA infection. To test our hypothesis, we will first determine if migration to lymph nodes is required for LCs to induce MHC-II-dependent inhibition of oral Tc17. Second, determine if IL-6/STAT3 or TGF-? are required to drive Tc17 persistence and plasticity to Tc1 in the oral mucosa in vivo. To the best of our knowledge, we are the first group to define MHC-II presentation on LCs as key factor in Tc17 expansion in the oral mucosa. We optimized interstitial leukocytes sorting, enumeration and phenotype assessment from the oral mucosa excluding blood or nasal- associated lymphoid tissues. Critically, we can unequivocally and permanently mark the phenotype and developmental fate of IL-17A-expressing CA-specific CD3+ T cells assessing their phenotypic plasticity. Our strategy employs genetic crossing of reporter- and silencing- murine strains. We can use MHC-I and MHC-II tetramers capable of uniquely recognizing CD8+ and CD4+ T cells specific for an epitope-tagged? CA strain. We expect to determine the role of LCs as regulators of the developmental re-programming CA-specific Tc17 to Tc1 cells either in the oral mucosa or cervical lymph nodes. With a deeper understanding of the mechanisms initiating oral candidiasis comes opportunities through therapeutic interventions to manipulate key steps and mitigate or prevent systemic candidiasis.